1. Field of the invention
The present invention relates to a system for regulating the operating point of a direct current power supply comprising a current generator system connected to a pulse width modulation converter.
2. Description of the prior art
Electrical power is usually supplied to an aircraft or spacecraft from current generators such as solar generators. Current generators have a substantially rectangular output current-voltage characteristic I(V), acting as a current source in the area (I) and a voltage source in the area (II) as shown in FIG. 1a. The output power-voltage chracteristic P(V) is substantially triangular, as shown in FIG. 1b.
These generators are normally associated with an electrical energy converter using pulse width modulation to deliver rectangular voltage pulses the width of which varies according to the power consumed by a load circuit. This type of converter is usually referred to as a "PWM" converter and is used in devices referred to as "BUCK", "BOOST" or "BUCK-BOOST" devices.
The input current-voltage characteristic I(V) of a converter of this kind supplying a load consuming constant power is in the shape of the positive part of an equilateral hyperbola as shown in FIG. 1c as these essentially reactive and highly efficient converters consume very little power.
Conventionally, the electronic regulation loop of these converters includes an error amplifier comparing the voltage to be regulated (the voltage supplied to the load) with a reference voltage, the amplified error signal being supplied to a comparator which modulates the width of the voltage pulses supplied by the converter by comparing the error signal with a signal generated by a sawtooth signal generator. An integrator is included at the output of the comparator to provide a null static error.
Depending on the direction in which the error signal is varying as a function of variation in the voltage to be regulated, a given converter has its operating point either in the current source area I or in the voltage source area II of the output current-voltage characteristic of the generator for a constant consumed power P, as shown in FIGS. 1d and 1e.
As the power drawn by the load increases the aforementioned operating point moves gradually along the output characteristic I(V) of the generator towards a point at the maximum power P.sub.max that can be supplied by the converter.
If the power drawn exceeds the maximum power P.sub.max the operating point previously situated in the current source area or in the voltage source area goes to the voltage source or the current source area as shown in FIGS. 1f and 1g beyond or short of the point with coordinates I(P.sub.max), V(P.sub.max). Under these conditions the operating point becomes unstable because there is a change of operating conditions, that is to say a change from the current or voltage source area to the other.
Because of its instability the operating point moves as far as the point on the output current-voltage characteristic I(V) characterized either by I=0 or V=0 and which corresponds to null power supplied by the solar generator. This is the phenomenon of "stalling", a stable state as shown in FIGS. 1f and 1g.
A system for extracting maximum power from a direct current generator with a substantially rectangular characteristic I(V) is described in French patent application No. 2 031 063. This system includes a loop controlling a transistor in the converter at variable frequency. It is not possible to obtain a regulated voltage with this system.
An object of the system in accordance with the invention for regulating the operating point of a direct current power supply is to remedy the aforementioned disadvantage by eliminating the stalling phenomenon.
Another object of the present invention is a regulation system for a direct current power supply in which the amplitude of excursion of the operating point about the maximum power point P.sub.max is variable.
Another object of the present invention is a regulation system for a direct current power supply in which when the power drawn is less than the maximum power P.sub.max the operating point may be varied either on the current source characteristic area or on the voltage source characteristic area.
A final object of the present invention is a regulation system for a direct current power supply in which a solar generator can be connected to the converter with no special precautions, one of the operating points corresponding to the power actually drawn being automatically achieved.